I. Field of the Invention
The present invention relates generally to heated element gas flow sensors and, more particularly, to a method and system for compensation of contamination of the heated element in the gas flow sensor.
II. Description of Related Art
Gas flow sensors which measure the mass gas flow are utilized in many different applications. For example, gas flow sensors are utilized in the intake manifold of automotive vehicles to measure the mass airflow through the intake manifold and into the engine. Similarly, gas flow sensors are utilized to measure the mass flow of flammable gases, such as propane and the like, in both automotive as well as other applications, such as fuel cell applications.
Many of the previously known gas flow sensors utilize a heated element to determine the gas flow rate through the sensor. When used in an automotive application, this flow rate, in conjunction with other parameters, is then coupled as an input to a fuel management system which regulates and controls the operation of the engine for improved engine performance, reduced emissions and the like.
These previously known heated element gas flow sensors utilize a resistive element which is disposed within the gas flow path of the sensor. A control circuit varies the current flow through the resistive element by an amount sufficient to maintain the temperature differential between the resistive element and ambient air temperature at a predetermined constant, typically 200xc2x0 centigrade. The magnitude of current through the resistive element is then proportional to the gas flow rate through the flow sensor.
While these gas flow sensors accurately determine the gas flow rate through the sensor when the gas flow sensor is relatively new, after prolonged use, the heated element may be subjected to contaminants of one sort or another. For example, when the gas flow sensor is utilized in the intake manifold of an automotive engine, the heated element is subjected to dirt, dust, oil and other debris in the air stream inducted through the gas flow sensor. After prolonged use, this debris can and does adhere to the outer surface of the heated element.
When the heated element becomes increasingly covered with debris and other contaminants, the thermal capacitance of the heated element increases. This increase in thermal capacitance, furthermore, results in an inaccurate output signal from the gas flow sensor. Such an inaccurate signal, in turn, may result in poor engine performance, increased emissions and other undesirable effects.
The present invention provides both a method and system for compensation of contamination of a heated element in a heated element gas flow sensor which identifies not only the degree of contamination of a heated gas flow sensor, but also compensates for this contamination to provide a more accurate signal to the fuel management system of the engine and thereby enhance engine performance and achieve low emissions.
In brief, the system of the present invention provides for compensation of contamination of a heated element of the type where the heated element and a cold element are disposed within the gas flow. Upon engine startup, electrical power is applied to the heated element so that the heated element rises in temperature from an ambient temperature and to an elevated temperature during a startup period. Once the heated element achieves its elevated temperature, typically 200xc2x0 centigrade above ambient temperature, a control circuit for the heated element gas flow sensor maintains the temperature differential between the heated element and the cold element of the gas flow sensor at a constant value by varying the current through the heated element. The magnitude of the current flow through the heated element is thus proportional to the gas flow rate through the flow meter.
The system of the present invention includes a processing circuit having an input and an output. A voltage signal from the heated element is connected to the processing circuit input. Preferably, the processing circuit comprises a microprocessor based processing circuit.
Means in the processing circuit then read a plurality of temporally spaced signals corresponding to the temperature of the heated element as data from the input to the processing circuit during the startup time period. Typically, this data forms a curve having a defined slope as the heated element becomes heated from an ambient gas temperature and to the elevated steady state operating temperature.
The processing circuit is programmed to compute parameters of a transfer function corresponding to the data. Preferably, this transfer function is a second order transfer function and is computed on a real time basis during the startup time period following the application of electrical power to the heated element using an adaptive algorithm.
After the processing circuit computes the transfer function parameters the processing circuit determines a contamination correction factor as a function of at least one parameter of the transfer function. In practice, the slope of the transfer function parameter constitutes the most important factor of the transfer function and this slope is then utilized by the processing circuit to generate the correction factor. The processing circuit then utilizes the correction factor to modify the measured gas flow rate by the gas flow meter after the heated element attains the elevated steady state temperature to compensate for contamination of the heated element.